1. Field of the Invention
The present invention relates to a method of calibrating a CCD camera. More particularly, the invention relates to a method of calibrating distortion of a solid camera and a lens system built in a high-resolution CCD sensor.
b 2. Description of the Prior Art
Reflecting the significantly improved resolution of CCD cameras as a result of the increased number of picture elements built in each solid sensor, recently, demand for those high-resolution CCD cameras has sharply grown as image sensors for picking up image of processed objects in those processes for manufacturing, assembling, and inspecting a variety of automotive parts and bodies. Image data picked up by each CCD camera are eventually processed by a computer system via an image processing unit, and then, based on those processed data, appearance of the processed parts and bodies is finely measured and inspected.
Each picture element built in a solid sensor of a CCD camera has several microns of magnitude. When operating a CCD camera incorporating 1000.times.1000 units of picture elements under a certain condition, the CCD camera measures the appearance of the objective body in micron order. Nevertheless, if there were even the micron-order distortion in the arrangement of picture elements in a solid sensor normally containing a large number of picture elements respectively being aligned in a latticed formation or in the arrangement of a lens system projecting light reflected from an objective body against the solid sensor, or if there were fine deviation in the assembly of the solid sensor and the lens system, then, unwanted distortion will be generated in those images present in specific domains corresponding to the above-cited distorted or deviated spots, thus eventually generating error that adversely affects the measured value beyond an allowable scope.
For example, see FIG. 4. Assume that there is such a CCD camera 1 which is capable of correctly functioning itself in terms of performance accuracy, where light beam from point "P" on a visual line "b" of angle ".alpha." against optical axis "a" of a lens system 4 is incident upon a picture element 3m built in a solid sensor 2. Also assume that there is distortion in the lens system 4 to cause the light beam from the point "P" to be incident upon another picture element 3n adjoining the picture element 3m. Then, since there is distortion in the lens system 4, the CCD camera 1 incorrectly identifies that there is a point "P" in the direction of a visual line "c" designated by a dot-chain line connecting the picture element 3n to the principal point Q of the lens system 4. This is because angles ".alpha." and ".alpha.'" between the optical axis "a" and the visual line "b" and between the optical axis "a"0 and the visual line "c" are respectively different from each other. In consequence, the farther the distance from the point "P", the poorer the precision of the measured value of the three-dimensional coordinate of the point "P".
Therefore, whenever operating such a CCD camera requiring extremely high precision, it is essential for the inspection system to calibrate the CCD camera beforehand in order to correct the measured value which is adversely affected by the distortion cited earlier. To implement this, as shown in FIG. 5 for example, a calibration method by rotating a CCD camera in the vertical and horizontal directions has been developed. FIG. 5 schematically illustrates the calibration method cited above, where an objective CCD camera is secured onto a swingable base 10, which is capable of rotating the CCD camera in the vertical and horizontal directions by way of pivoting on the principal point Q of a lens system 4. Specifically, using a rotary encoder (not shown) available for measuring angle of the rotation of the CCD camera in the vertical and horizontal directions, the objective CCD camera is rotated in the vertical and horizontal directions. The above-cited conventional calibration method is described below.
A light source 11 is disposed at a predetermined position in front of the objective CCD camera 1. The objective CCD camera 1 is rotated in order that light beam from the light source 11 can be incident upon a picture element 3s in the principal of a solid sensor 2 built in the objective CCD camera 1. In this case, the light source 11 is exactly in accord with the optical axis "a" of the objective CCD camera 1, and thus, the direction of the optical axis "a" is determined. Next, as shown in FIG. 5 with a dot-chain line for example, the CCD camera 1 is rotated in the upward direction by such an amount corresponding to angle ".alpha." by way of pivoting on the principal point Q of the lens system 4. If the CCD camera 1 were equivalent to the one shown in FIG. 4 exerting normal performance accuracy, then, light beam from the light source 11 is incident upon a picture element 3m of the solid sensor 2. On the other hand, if there were any distortion in the lens system 4, then, light beam from the light source 11 is incident upon another picture element 3n. Therefore, even when light beam is incident upon either the picture element 3m or the other picture element 3n, if only such a corrective process is executed to deal with this incident light as the one which is incident upon either of these picture elements at a predetermined angle ".alpha." against the optical axis "a", then, such an incorrectly measured value caused by the distortion in the lens system 4 can properly be corrected. The calibration method mentioned above is repeatedly executed by rotating the CCD camera 1 in the vertical and horizontal directions by gradationally varying angle against the optical axis "a". After completing the above calibration process, the CCD camera 1 is effectively made available for measuring appearance of processed parts and bodies based on the calibrated data with extremely high precision.
The conventional calibration method described above by way of rotating the objective CCD camera can easily and properly provide three-dimensional angle of rotation of the CCD camera by applying a rotary encoder. Nevertheless, it is extremely difficult for any conventional calibration system to properly rotate the objective CCD camera merely by way of pivoting on the principal point of the lens system. In other words, the lens system comprises a variety of lenses which are combined with each other, and yet, the principal point of the lens system is substantially an optical ideal point which cannot precisely halted. Furthermore, even though the CCD camera can correctly pivot on the principal point of the lens system, from the viewpoint of micron-order, it is quite difficult for the conventional calibration system to correctly maintain the relationship between the angle of the incident light against the optical axis and the corresponding picture elements built in the solid sensor of the objective CCD camera. In consequence, any of the conventional calibration systems can hardly achieve a calibration process with extremely high precision. Furthermore, although precision in the calibration of the objective CCD camera can be promoted by rotating the CCD camera on the second basis or by applying an extremely fine angle below the second basis, it is technically quite difficult to correctly and stably rotate the objective CCD camera by strictly observing such an extremely fine angle. In consequence, these adverse factors obstruct any of those conventional systems to securely calibrate the objective CCD camera with extremely high precision.